The present invention relates to an overpressure valve for a hydraulic pit prop comprising a housing, a valve seat formed in the housing, a valve body movable in the housing between a closed position engaging the valve seat and an open position, and a gas cushion provided between the valve body and the housing and arranged to bias the valve body to the closed position.
In known overpressure valves of the aforementioned type (German Pat. No. 1,199,575 and German Offenlegungsschrift No. 1,583,109) the gas under pressure of the gas cushion diffuses after a certain period from its chamber, resulting in a drop of pressure of the gas cushion, which, in turn, results in a drop of the predetermined nominal loads of the pit props. In extreme cases this will result that the pit prop no longer has an adequate nominal load capacity.